Someone I know who has been playing electric guitar for many years says this quite often, as a key difference between playing acoustic and electric guitar. He claims that even playing rhythm, you shouldn't be strumming all 6 strings but only two or three (typically by playing power chords).
I can see this having a point with certain amp/effect setups - it gets awfully muddy - but I'm not convinced it should be considered a rule. Isn't there a place for playing an electric with standard open acoustic guitar chords?
I'm very new to electric guitar so I'm trying hard to determine which things I'm told should be considered rules, and which is just that person's style of play.